(i) Field of the Invention
The present invention relates to a liquid crystal optical element utilizable in a display for displaying characters, figures and the like, a dimmer in which the transmission quantity of an incident light changes, an optical shutter and the like. More specifically, it relates to a liquid crystal optical element in which a smectic liquid crystal material is used.
(ii) Description of the Related Art
As a liquid crystal display in which a wide viewing angle and a rapid response can be expected, there has been suggested a display element using an optical switching phenomenon of a ferroelectric liquid crystal material by N. A. Clark and S. T. Lagerwall (App. Phys. Lett., Vol. 36, p. 899 (1980) (hereinafter referred to as "Reference 1"). A surface-stabilized ferroelectric liquid crystal (SSFLC) optical element is characterized in that an optical response is rapid (1 msec or less) and a viewing angle is wide. However, the SSFLC is bistability, and the electro-optical response of the SSFLC optical element is limited to the switching of two states of a light state and a dark state. Therefore, the SSFLC has a problem that it is difficult to display a half tone by the control of a voltage. Furthermore, it also has a problem that the liquid crystal layer is liable to form a chevron structure, so that a contrast is low and a layer structure is easily disturbed by a mechanical impact, and what is worse, the recovery of a once disturbed orientation is difficult.
As another optical element, a liquid crystal material having an antiferroelectric phase (an antiferroelectric liquid crystal material) has been reported by Chandani et al. (Jpn. J. Appl. Phys., Vol. 28, L1265 (1989); hereinafter referred to as "Reference 2"), and a display element utilizing the antiferroelectric liquid crystal material has also been suggested (Jpn. J. Appl. Phys., Vol. 29, p. 1757 (1990); hereinafter referred to as "Reference 3"). The antiferroelectric liquid crystal material has tristability based on a phase change between an antiferroelectric phase and a ferroelectric phase, and by the utilization of a mechanism of switching them under the application of a bias voltage, a display device which permits the display of the half tone can be manufactured. However, in the display element in which the bias voltage is necessary for the display of the half tone and a high precision and many scanning lines are required, there is a problem that a drive waveform is complex.
On the other hand, a smectic liquid crystal material (hereinafter referred to as "the thresholdless smectic liquid crystal material") in which a curve showing a relation of a light transmittance to an applied voltage has a V-shaped form (V-shaped characteristics) has been reported by Inui et al. and Tanaka et al. (The 21st Liquid Crystal Debate Lecture Textbook 2C04, p. 222 (1995) and p. 250 (1995); hereinafter referred to as "Reference 4"). It is described therein that the thresholdless smectic liquid crystal optical element using this material does not possess any definite threshold for a phase change and has low hysteresis characteristics.
Furthermore, a liquid crystal material having a diagonal line of 5 inches which comprises a combination of this thresholdless smectic liquid crystal optical element and a thin film transistor (TFT) has been reported by T. Saishu et al. [SID'96 Digest, p. 703 (1996); hereinafter referred to as "Reference 5"]. In this report, there is first used, as a test cell, a cell having a cell gap of 2 .mu.m and an electrode area of 1 cm.sup.2 into which a thresholdless smectic liquid crystal material TLAF-1 made by Mitsui Petrochemical Industries, Ltd. is poured. It is described that when an auxiliary capacity (a storage capacity) is increased and a temperature is raised by this cell, the voltage holding properties of the liquid crystal can be improved. However, in the liquid crystal element having a diagonal line of 5 inches which is combined with an actually manufactured thin film transistor, a contrivance for the increase of the auxiliary capacity and the like obtained in the test cell is not carried out. Therefore, in an AC type drive in which writing is done at both of positive and negative polarities, a contrast ratio is less than 10, and hence it cannot be sufficiently secured. In consequence, a DC type drive in which the writing is done with one polarity is employed. By this technique, the liquid crystal optical element having 234 scanning lines and a diagonal line of 5 inches is subjected to an NTSC drive (a writing time at a gate is 63.5 .mu.s), thereby obtaining a contrast ratio of 10 or more.
As described above, it is possible to drive the thresholdless smectic liquid crystal optical element by an active element such as the TFT. However, the above-mentioned thresholdless smectic liquid crystal material has a large spontaneous polarization value of 100 nC/cm.sup.2 or more, and in order-to drive the thresholdless smectic liquid crystal material having such a large spontaneous polarization, it is necessary to pour an electric charge in proportion to the spontaneous polarization. However, since a charge quantity which can be fed from the TFT is limited, there is a problem that the number of the scanning lines of the drivable liquid crystal optical element is decreased. In fact, in Reference 5, the number of the scanning lines is 234, which is less than the half of a usual VGA type. Furthermore, as shown in Reference 5, some problems are present. For example, in order to obtain the sufficient contrast ratio, a sufficient writing charge quantity is necessary, and in order to secure the charge quantity, there is employed the DC drive in which the writing is done under a unipolar voltage instead of a bipolar voltage (an AC type), so that burning takes place. Additionally, even in the case of the DC drive, in a writing time in one frame, the writing charge quantity is insufficient, and in order to secure the sufficient charge, the charge pouring of several frames is necessary, so that a response time of the liquid crystal optical element is prolonged inconveniently.
As one of means for solving these problems, there is a technique of adding a large auxiliary capacity to the TFT as shown in Reference 5, but if the auxiliary capacity is increased, an aperture efficiency ratio of the liquid crystal optical-element deteriorates, so that the display is in a dark state. Furthermore, according to the investigation of the inventors, the increase of the auxiliary capacity leads the increase of an RC constant, and in order to perform the sufficient writing within a predetermined writing time, it has been understood that lowering an ON-state resistance R of the TFT and improving TFT characteristics are necessary. That is to say, if an ON-state current of the TFT cannot be sufficiently obtained, the writing is not completed within the writing time, because of the increased capacity value. Therefore, when the TFT characteristics are decided and when the spontaneous polarization value of the liquid crystal material and a panel structure are decided, an optimum limit value of the auxiliary capacity is present, and the auxiliary capacity larger than the limit value increases the RC constant and decreases the poured charge quantity within the writing time, with the result that the writing charge quantity to the liquid crystal is reduced.
The drive of the liquid crystal material having the large spontaneous polarization can be accomplished by using the TFT having the sufficient characteristics or by doing the writing at a high voltage. In this case, however, the following problems exist. In the first place, the development of the new TFT having the sufficient characteristics is necessary. Next, it is necessary to develop a drive circuit which can apply a high drive voltage. Even if these two intentions are realized, it is required that a large amount of the charge is allowed to flow in order to drive the liquid crystal material having the high spontaneous polarization, so that an extremely large power is inconveniently consumed.
On the other hand, it can easily be presumed that if the spontaneous polarization of the thresholdless smectic liquid crystal material is decreased, the above-mentioned problems can be solved. However, there have not been reported a smectic liquid crystal material having electro-optical characteristics that a curve showing a relation of a light transmittance to an applied voltage has a V-shaped form, and having the low spontaneous polarization.